Fukushima Radiation in Migrating Bluefin Tuna Expected to Fall

May 29 (Bloomberg) -- Radioactive material found in bluefin
tuna that swam or fed in waters off the crippled Fukushima
nuclear plant in Japan is likely to decrease over time as the
material dilutes in the ocean, scientists said.

A study of 15 Pacific bluefin caught off San Diego in
August last year found levels of radioactive cesium 10 times
higher than in fish caught in previous years and provide
“unequivocal evidence” that the radiation came from Fukushima,
researchers including Daniel Madigan and Nicholas Fisher said in
a study published yesterday in the Proceedings of the National
Academy of Sciences.

Contamination levels, which the authors say are not a
danger to public health, are likely to decline even though
cesium has ‘biomagnification’ characteristics, meaning the
concentration increases from prey fish to predators when cesium
is consumed. That would be offset by the bluefin’s metabolism,
which should excrete cesium at a rate of about 2 percent per
day, Fisher wrote in an e-mail.

“Much will depend on the concentration in the prey fish,
which in turn is ultimately dependent on the water
concentration,” wrote Fisher, a professor at Stony Brook
University in New York, in response to e-mail questions. “If
concentrations in water will eventually decline, as we would
expect, due to dilution and dispersion, then concentrations in
living organisms will eventually decline as well.”

Prized Sushi

The bluefin is the most prized among tuna for making sushi
or sashimi in Japan, where a 5-centimeter (2-inch) piece of the
fattiest part of the fish, called otoro in Japanese, can cost
over 1,000 yen ($12.58) in a Tokyo restaurant.

Bluefin can grow as long as 3 meters (9.8 feet), and have
been proven to travel as far as 45,000 miles in 16 months,
according to Tagging of Pacific Predators, a research program
co-managed by Stanford University’s Hopkins Marine Station.

The group plans to conduct another bluefin tuna study this
year to determine whether cesium concentrations have changed,
Fisher wrote.

“This probably shows the need for international monitoring
of marine life,” Japan’s Chief Cabinet Secretary Osamu Fujimura
told reporters today in Tokyo. “We are studying the best way to
collect information on the issue.”

About 3,500 terabecquerels of cesium 134 and 3,600
terabecquerels of cesium 137 may have leaked into the sea
between March 26 and Sept. 30 last year after the meltdown of
three reactors at the Fukushima Dai-Ichi nuclear plant, operator
Tokyo Electric Power Co. said in a statement last week.

Weapons Testing

The study compared bluefin with fish caught before the
disaster, which showed no trace of cesium 134 and only
background levels of cesium 137 from nuclear weapons testing.

Cesium levels found in the post-Fukushima bluefin were less
than the radioactive doses naturally occurring in the fish from
isotopes including potassium 40, the study found.

“It is unlikely” that the level will rise in tuna, Daniel
Madigan, the study’s co-author and researcher at Hopkins Marine
Station at Stanford, wrote in an e-mail.

“However, certain small fish around Japan showed very high
levels after the accident. If certain larger predators happen to
feed on these prey, higher levels than we observed may be
possible.”